Electro-optical module comprising flexible connection cable and method of making the same

ABSTRACT

An electro-optical module comprising flexible connection cable and aligning capabilities is disclosed. Electro-optical devices may be soldered on a transparent substrate such as glass or a substrate comprising an optical waveguide wherein electrically conductive traces are designed, forming an electro-optical module. When such electro-optical module is inserted and aligned into a printed circuit board, the external part of the substrate, comprising electrically conductive traces and pads, referred to as flex-cable, is bent down toward the mounting plane of the PCB allowing to establish electrical connections between these pads and the PCB. The substrate may be brokenalong a pre-formed groove, and the external part of the substrate can be removed leaving the flex-cable section in place.

FIELD OF THE INVENTION

The present invention relates generally to optical communicationapparatus and more specifically to the implementation of opticalinterconnect links in printed circuit boards.

BACKGROUND OF THE INVENTION

Typically systems that might potentially use optical interconnectswithin the next few years are servers, supercomputers and telecomswitch-routers, comprising multiple racks, wherein each rack has abackplane or midplane, and multiple plug-in cards (or blades).Communication between the plug-in-cards is routed through the backplane.The aggregate data rate within a single rack is expected to reachmultiple Terabit per second (Tb/s) by the end of the decade, and it isassumed that it will grow to tens of Tb/s in the future. Currentlycommunication is achieved electrically through copper lines. However, inelectrical lines the attenuation and cross-talk increase with frequency.For board to board interconnects with a typical distance of one meterthe crossover point where optical transmission becomes more powerefficient than electrical transmission is at 5 Gb/s. Furthermore,optical transmission lines can be packed much denser than electricallines.

The reliable, accurate and cost-effective integration of opticalelements such as waveguides, transmitters and detectors in PCB's is achallenge lying mainly in high integration and optical alignment. Thealignment requirements of optical elements are much tighter than thoseof electrical components. For example, the minimum alignment tolerancein the realization of a printed circuit board is 20 μm, optical elementsneed to be positioned with an accuracy of 5 μm (for 50 μm×50 μm squareoptical waveguides). Consequently, it is a challenge to align opticalelements in a printed circuit board.

SUMMARY OF THE INVENTION

Thus, it is a broad object of the invention to remedy the shortcomingsof the prior art as described here above.

It is another object of the invention to provide a high integrationelectro-optical module.

It is a further object of the invention to provide a method and systemfor aligning high integration electro-optical modules.

It is still a further object of the invention to provide a method andsystem for manufacturing high integration electro-optical modules.

To accomplish these and other related objects, the invention provides anelectro-optical module adapted to be connected to a printed circuitboard, the electro-optical module comprising a stiffener with at leastone electrically conductive trace, characterized in that a portion ofthe stiffener, comprising a portion of the at least one electricallyconductive trace, is removable and adapted to establish an electricalcontact between the electro-optical module and the printed circuitboard.

The invention also encompasses a method for manufacturing anelectro-optical module adapted to be connected to a printed circuitboard, the electro-optical module comprising a stiffener with at leastone electrically conductive trace, a portion of the stiffener comprisinga portion of the at least one electrically conductive trace beingremovable and adapted to establish an electrical contact between theelectro-optical module and the printed circuit board, the methodcomprising the steps of:

coating dielectric material on top side of the electro-optical module,

depositing and patterning of metal material,

applying second layer of dielectric material, and,

opening the second layer of dielectric material at predeterminedpositions.

The invention further encompasses a method for mounting anelectro-optical module adapted to be connected to a printed circuitboard, the electro-optical module comprising a stiffener with at leastone electrically conductive trace, a portion of the stiffener comprisinga portion of the at least one electrically conductive trace beingremovable and adapted to establish an electrical contact between theelectro-optical module and the printed circuit board, the printedcircuit board having a mounting plane comprising a cavity adapted toinsert partially the electro-optical module, the method comprising thesteps of:

inserting, aligning, and fastening the electro-optical module in thecavity of the printed circuit board,

bending down the portion of the stiffener comprising a portion of the atleast one electrically conductive trace toward the mounting plane,

establishing an electrical connection between the at least oneelectrically conductive trace of the portion and the mounting plane.

These and other aspects of the invention are described in further detailbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a partial perspective view of an electro-opticalsystem wherein the invention can be implemented.

FIG. 2 is a partial cross section view of the printed circuit boardrepresented on FIG. 1 wherein the invention is implemented.

FIG. 3, comprising FIGS. 3 a, 3 b, 3 c, and 3 d, illustrates an exampleof an electro-optical module according to the invention.

FIG. 4 shows the soldering of an electro-optical module flex-cablewithin the pads of a PCB that also hosts one or more electronic orelectro-optical modules.

FIG. 5 illustrates an example of the steps of a process allowing themanufacturing of the electro-optical modules of the invention usingstandard semiconductor manufacturing tools and processes.

FIG. 6 shows a further embodiment illustrating the use of silicon formaking the stiffener of the electro-optical modules of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention introduces a design technique to be used inmanufacturing a highly integrated electro-optical module with very smalldimensions. According to the invention there is provided means formanufacturing, aligning, and connecting such modules. The number ofprecise alignment steps, where optics is engaged with the board, isreduced to an absolute minimum, one.

For sake of illustration the description is based upon Vertical CavitySurface-Emitting Laser (VCSEL) as well as the associated optical signaldetector that need to be assembled into small cavities like the one thatcan be created into optical back-planes or can be used to connectelectrical signals to Micro ElectroMechanical Systems (MEMS) or MicroOpto-ElectroMechanical Systems (MOEMS).

These modules are designed to allow assembling of one or more devicesonto a thermally stable structure with high performance electricalproperties. The mounting operations are performed on flat surfaces tomake these operations compatible with most of the industrialinfrastructure already available and with no added costs. The modulescan be folded and assembled in 3D shapes based on the systemrequirements with simple operation of breaking and removing the rigidstiffener that is used to assemble the module itself.

FIG. 1 illustrates a partial perspective view of an electro-opticalsystem 100, such as a server, a supercomputer or a telecomswitch-router, wherein the invention may be implemented. For sake ofillustration electro-optical system 100 consists in a rigid back-plane105, a flexible back-plane 110, and a Printed Circuit Board (PCB) 115connected to both back-planes 105 and 110 through optical connectors120-1 and 120-2. PCB 115 comprises two semiconductor devices 125-1 and125-2 and several electro-optical modules 130-1, 130-2, and 130-3. Inthis implementation example, optical signals are transmitted fromelectro-optical module 130-2 to electro-optical module 130-1, fromelectro-optical module 130-1 to optical connector 120-1, and fromelectro-optical module 130-3 to optical connector 120-2. Electro-opticalmodules 130-1, 130-2, and 130-3 are manufactured, aligned, and connectedto PCB 115 according to the invention. Therefore, electro-optical module130-1 combines a Vertical Cavity Surface-Emitting Laser and an opticalsignal detector, electro-optical modules 130-2 and 130-3 carry VerticalCavity Surface-Emitting Laser. In general, each electro-optical modulewill contain an array of Vertical Cavity Surface-Emiting Lasers, anarray of optical detectors or both. Such arrays are available on one diethat contains either VCSEL's or detectors, where the VCSEL's anddetectors have a well defined separation which is for example 250 μm.The VCSEL and detector arrays may be arranged in a one dimensionallayout e.g., 1×4, 1×8, or 1×12, or a two dimensional layout e.g., 2×4 or4×12.

FIG. 2 is a partial cross section view of the PCB 115 of FIG. 1,according to line A-A′, illustrating the structure of electro-opticalmodules 130-1 and 130-2, generically referred to as 130, as well as theprinciples used for their alignment and connection. As shown, in apreferred embodiment, electro-optical module 130 comprises a holder 200and a plurality of alignment pins 205 that fit to PCB holes so as toalign electro-optical module 130 on PCB 115. Several dielectric andelectrically conductive layers 215 are formed on one surface of thestiffener 210 of the electro-optical module 130. For example, thestiffener 210 can comprise one electrically conductive layer, sandwichedbetween two dielectric layers, wherein electrically conductive tracksare designed. In the example given on FIG. 2, the semiconductor devicesof the electro-optical module 130 are soldered to pads belonging to thiselectrically conductive layer. The electro-optical module 130 comprisesa surface emitting laser 220 and a driver 225 or an optical signaldetector 220 and an amplifier 225. Naturally, stiffener 210 is eithertransparent, or translucent, so that optical signal can cross it, orcomprises a waveguide 230. Still in a preferred embodiment,electro-optical module 130 further comprises a heat spreader and opticalshielding part 235. Electro-optical module 130 is butt coupled towaveguide 240 designed in PCB 115. When electro-optical module 130 isinserted in PCB 115 and aligned, the optical signal emitted from orreceived to semiconductor device 220 is transmitted through waveguide240. Electro-optical module 130 is electrically connected to PCB 115,using the set 245 of electrically conductive tracks and pads. Theelectrically conductive tracks and pads are implemented in a flexiblecable to overcome the 90 degrees angle between the stiffener 210 and thesurface of the PCB 115. In order to align the electro-optical module inthe board, a two step process is preferably used:

a receptacle, or holder, is accurately positioned in the PCB based onthe standard selective laser drilling concept, it may be inserted fromthe top or the bottom,

the electro-optical module contains etched reference structures e.g.,holes or trenches, that are the mirror of alignment features in theholder. A tight fit of the electro-optical module in the holder ensuresprecise alignment. Proper design enables multiple subsequent connectionsinto the same holder.

FIG. 3, comprising FIGS. 3 a, 3 b, 3 c, and 3 d, illustrates an exampleof an electro-optical module 130 according to the invention. FIG. 3 a isa front view of the module while FIGS. 3 b, 3 c, and 3 d are side viewsof the module before its insertion, during its connection, and when itis installed, respectively. Printed circuit board is not shown for sakeof clarity. The module stiffener 210 made of e.g., glass or siliconcomprises one or more layer of dielectric materials and electricallyconductive traces e.g., 300, to carry electrical signals, ending withpads e.g., 305. The electrically conductive circuit can also be coveredby a further layer of dielectric material. In the specific embodimentthe module at construction phase has three separate zones, one (I) forhosting devices, one (II) for the mechanical holder (alignment) and one(III) for the electrical connections. In a preferred embodiment, agroove is performed in the stiffener 210 between the second and thirdzones, as shown with dotted lines, so as to facilitate breaking of zoneIII of the stiffener 210. Such groove can be a pre-sawed channel. Theelectrically conductive traces and the pads of this third zone form atop flex-cable that is used to connect electro-optical module 130 to PCB115.

In this example, the electro-optical and electrical elements areflip-chip bonded on to the pads in zone one of the module. The opticalsignals travel through the module substrate. consequently, the substratemust be transparent e.g., glass. For the case of glass substrate, thesignals need preferably to be imaged through the glass using lenses inorder to obtain a high optical coupling efficiency and to preventoptical cross-talk. The stiffener 210 can host optical lenses 310 and/orlenses can be milled in the glass itself.

Once the assembled optical module represented on FIGS. 3 a and 3 b isinserted and aligned into the PCB cavity, and held into position byholder 200 and alignment pins 205, the part of the stiffenercorresponding to the third zone i.e., the part containing the topflex-cable, is bent down toward the mounting plane of the board as shownon FIG. 3 c, allowing the electrical connection e.g., soldering, of thepads 305 through e.g., a standard hot-bar or laser process. Thestiffener gets broken along the groove, or pre-sawed channel, and thestiffener portion can be removed, peeled-off, from the flex-cablesection as shown on FIG. 3 d.

Therefore, according to the invention, the path for high speedcommunication between the electronics hosted on the PCB and themicro-optical module (flex-cable) is the shortest.

In another embodiment, the flex-cable is soldered within the pads of thePCB 115 that also host electronic or electro-optical module 400 as shownon FIG. 4. Such technique allows the high speed signal to travel along asingle electrical media, avoiding impedance mismatch due to change ofphysical properties of the material surrounding the electricalconductor. The high speed signals are directly “injected” into theflex-cable as soon as possible, without the need to have the samesignals travelling through different media and through different levelof electrical interconnections prior to reach the optical activeelements. This is possible thanks to openings in the dielectric toplayer of the flat cable than can be achieved by standard laserprocessing e.g., the cable polymer is ablated away at the position ofthe copper pads and enables the exposed pads for further usage onsoldering operations such as mounting an electronic modules on top ofthe flex portion of the electro-optical module.

FIG. 5 illustrates an example of the steps of a process allowing themanufacturing of the described electro-optical modules of the inventionwith standard semiconductor manufacturing tools. In this example, thestiffener is made of glass and several electro-optical modules aremanufactured in the same time on a single glass wafer. This processcomprises the steps of,

step 1: performing grooving on the backside of the glass wafer atpredetermined positions according to the electro-optical moduledisposition design on the glass wafer,

step 2: coating BenzoCycloButene (BCB) based Cyclotene 4026-46 (14 μm)on top side (Cyclotene is a trademark of the Dow Chemical Company),

step 3: applying seeding layer to enable copper deposition,

step 4: depositing and patterning resist on top of the seeding layer,

step 5: electroless plating of copper,

step 6: stripping resist,

step 7: applying second layer of Cyclotene 4026-46,

step 8: opening the top layer of Cyclotene 4026-46 at the positions ofthe copper pads.

Since the implementation of lenses is difficult and introduces criticalalignment of the electro-optical module in the PCB, a preferablealternative is to use silicon substrate wherein transparency is obtainedby realizing an optical waveguide through the silicon wafer. Compared tothe use of glass substrate, the optical signals are geometricallyseparated which prevents optical cross-talk and yields good couplingefficiency through butt coupling. The through silicon waveguide can beformed as follow. First holes are etched through the silicon wafer,second the silicon wafer is oxidized which forms a silicon dioxide filmthat acts as a waveguide cladding. Finally, the holes is filled with anoptically transparent waveguide material with a refractive index that islarger than that of silicon dioxide.

FIG. 6 shows an example of this last embodiment, illustrating the use ofsilicon for making the stiffener. In such case, the light emitted by theVCSEL is guided through a polymer waveguide. For the case of a detectormodule, the basic layout is the same but the VCSEL is replaced by adetector. As shown, the VCSEL or detector 600 comprises Copper pads 610on which solder 615 is applied. Pads 610 are surrounded by passivationlayer 605. The stiffener 620 is made of silicon that external surface625 is oxidized (SiO₂). The stiffener surface on which VCSEL or detector600 is soldered is covered with two layers of BCB 630 and 635 whereinCopper pads 640 are designed for VCSEL or detector soldering. Copperpads 640 belong to the electrically conductive layer built on top of thefirst BCB layer 630 and protected by the second BCB layer 635. A polymerwaveguide 645 is designed in the stiffener 620 to transmit light fromthe VCSEL to the PCB or from the PCB to the detector.

Another embodiment of the invention uses Liquid Crystal Polymers asdielectric in the different layers built-up in the electro-opticalmodule.

The utilization of glass, silicon or alumina substrates offer a quitegood thermal dimensional stability, greater than plastics or PCBmaterials that helps in maintaining precise alignment between theoptical units. Glass allows to use laser soldering, silicon allows IR(laser) soldering of the flat cable solder joints on the board withoutaffecting the whole assembly to thermal cycle (jeopardizing the opticalelements alignment) due to the assembly materials “relative” thermalexpansion properties.

The electro-optical module technology of the invention gives also thepossibility to “mount first” the electro-optical module and test itbefore to place it on the product in place of mounting the loosecomponents in a complex board with cavities and other passive opticaldevices (mirrors and fibers) with defects found at test requiringrework.

From an application point of view, the electro-optical module of theinvention brings the following advantages:

the optoelectronic module is realized based on standard IC technologycompatible process steps that offer a very good alignment precision (1μm) of the opto-electronic components and alignment features formounting the element in the printed circuit board.

the realization is preferably a wafer based process hence, manyelectro-optical modules can be manufactured simultaneously, bringingmajor cost savings.

the electro-optical module combines all required functions for theelectro-optical or electro-optical conversion in one element hence, onlyone critical alignment step is required to position this element in theprinted circuit board.

the opto-electronic and electronic components are flip-chip mounted onthe glass or silicon substrate such that the light travels through thesubstrate. The glass or the silicon forms a well defined and accuratelypositioned interface with the waveguides on the printed circuit board. Aseparate heatsink can be bonded on to the opto-electronic and electroniccomponents without affecting the optical coupling section.

the electrical flex-cable bypasses the need for an optical turningmirror. Consequently, the optical coupling is simplified whichpotentially leads to a higher optical coupling efficiency between theoptical pin and the waveguides on the printed circuit board.

the electro-optical module concept can be used for multilayer waveguidesystems i.e., several waveguide layers on top of each other.

While the description is based upon Vertical Cavity Surface-EmittingLaser (VCSEL) and the associated optical signal detector that requirethe use of transparent material, translucent material, or opaquematerial comprising light waveguide, it must be understood that theinvention can be implemented with other kinds of light emitting andreceiving devices that do not require such stiffener materialcharacteristics. For example, it is possible to implement the inventionwith optical signal detector device that light emitting or sensiblesurface is located at the opposite side of the electrical connectorse.g., pads.

Naturally, in order to satisfy local and specific requirements, a personskilled in the art may apply to the solution described above manymodifications and alterations all of which, however, are included withinthe scope of protection of the invention as defined by the followingclaims.

1. An electro-optical module adapted to be connected to a printedcircuit board, said electro-optical module comprising an opticallytransparent glass stiffener with at one electrically conductive trace,and at least one lens adapted to focus at least one light beamtransferred through said stiffener, characterized in that a portion ofsaid stiffener, comprising a portion of said at least one electricallyconductive trace, is removable from said module and said trace and saidtrace is adapted to establish an electrical contact between saidelectro-optical module and said printed circuit board.
 2. Theelectro-optical module of claim 1 further comprising a holder adapted toalign and maintain said electro-optical module in said printed circuitboard.
 3. The electro-optical module of claim 1 wherein said portion ofsaid stiffener is breakable along a preformed groove.
 4. Theelectro-optical module of claim 1 further comprising a heat spreader. 5.The electro-optical module of claim 4 wherein said heat spreader beingoptionally adapted to shield light.